Valve devices are widely used in mine workings to protect equipment against excess pressure. It is usual in mining equipment to adopt as the pressure fluid an emulsion containing about 97% water. Such a pressure fluid medium is relatively safe. In the case of roof supports, the associated pressure-relief valve devices have to cope with sudden explosive-like stress relief phenomena which occur in roofs of longwall mine workings when the roof can collapse, even partially, in a fraction of a second. Customary valve devices such as described in DE No. 2914981 are not capable of responding to and resisting such conditions. Special valves termed "rock burst valves" have been developed to provide the maximum possible pressure fluid outflow cross-section with the minimum delay. Such valves do not re-close until pressure in the working chamber of the prop has dropped below the predetermined relief pressure.
DE No. 2636794 describes a pressure-relief valve which has a spring-loaded tappet provided with two pistons acting in opposite directions actuated by pressure fluid in a working chamber. The piston which acts in the direction to close the valve has a smaller working area than the other piston which acts in the direction to open the valve. The valve then opens when the pressure exceeds a pre-determined threshold level set by the spring force and the relative piston working areas. The valve surfaces are provided on a fixed valve spring and a movable valve closure member. The seating is often made from plastics while the closure member and other components are made from steel. Plastics seatings are particularly resistant to dirt particles which inevitably contaminate the pressure fluid but nevertheless these seatings are sensitive to impact force. Such impact force occurs mostly when the closure member returns under spring force to re-establish the valve seating action. Repeated impacts causes premature wear in the seatings.
A general object of the invention is to provide an improved form of valve device for the aforementioned application which will provide an ample outflow of pressure fluid rapidly in the case of excess pressure yet protects the valve seating from damage on closure.